Interpretive Summary: Leaf rust is the most important disease of wheat worldwide. Wheat leaf rust resistance gene Lr12 is effective in adult plants, but not in seedlings. However, this gene can be effective in seedlings when the complementary gene Lr27 is also present. Our long term goal is to understand how Lr27 increases the effectiveness of Lr12. In this paper, we determined the chromosome location of Lr12 and identified molecular markers that flank the gene. These may be useful for further fine mapping studies that could eventually lead to isolating the gene.

Technical Abstract:
Wheat (Triticum aestivum) gene Lr12 provides adult-plant race-specific resistance to leaf rust caused by Puccinia triticina. It is completely linked or identical to Lr31, which confers seedling resistance only when the complementary gene Lr27 is also present. F2 and F2-derived F3 families were developed from a cross between the susceptible variety Thatcher and TcLr12, an isoline carrying Lr12. Of 230 F3 families, 55 were homozygous resistant, 115 were segregating for resistance, and 60 were susceptible to P. triticina, fitting a monogenic 1:2:1 segregation ratio. Lr12 was mapped on chromosome arm 4BL and was flanked by markers Xgwm251 and Xgwm149 at a distance of 0.9 cM and 1.9 cM, respectively. Using linked markers and wheat deletion stocks, Lr12 was located in deletion bin 4BL-5, FL = 0.86-1.0, comprising the terminal 14% of 4BL. The markers will be useful for following Lr12/Lr31 in crosses and for further mapping studies.